Feathering Edges in Node compositor
Adding a vignette to a render is fairly common and you can find examples in many tutorials. A vignette is often circular and darkens the corners but the same principal applies.

Basically you create a mask, blur it, then use it to mix together the results you want. In this example I have used a CombineRGBA to input a semi-transparent white background that I added a render result over. There is also an Ellipse Mask if you want the rounded result.